1. Technical Field of the Invention
The present invention relates to the inspection of manufactured devices and, more specifically, to an apparatus, system, and method for optically inspecting Printed Circuit Boards (PCBs) for manufacturing defects such as missing components, misplaced components, skewed components, and foreign objects.
2. Description of Related Art
Printed Circuit Boards (PCBs) are typically constructed from a heat-resistant plastic base with selected channels cut therein for holding metallic conductors, and are usually covered with an insulative resin. PCBs simplify the construction of electronic devices such as computers, smoke detectors, stereos and other consumer electronics by providing a common support for, and the correct electrical connections between, electronic components such as resistors, capacitors, inductors, and digital devices. A manufacturer need only correctly place an appropriate electronic component on a PCB in a designated location and, because the PCB is designed to provide the appropriate electrical connections between components, practically all electrical connections required between that device, other devices or output connections are automatically made. This allows for the "wireless" construction of many electronic devices, which greatly reduces manufacturing time and practically eliminates errors commonly associated with wire connections.
In the manufacturing of PCB based devices, the PCB is populated with components either by hand or by machine, and the components are then soldered to the PCB by methods well known in the art, such as with solder machines or with surface-mount technologies. The PCB is then inspected to insure that each component was correctly placed on the PCB. One method of inspection is optical inspection.
Optical inspection devices typically employ a PCB support which holds a populated PCB under an overhead camera and multiple light sources. In operation, the optical inspection device typically lights the PCB from several directions in order to eliminate shadows, and the camera captures a gray-scale (black-and-white) image of the PCB. In a process commonly called convolution, this image is then sent to a computer which compares the image, pixel by pixel, to a stored image of a properly populated PCB. Differences between the captured image and the image of the properly constructed PCB are recorded as defects.
The existing method of optically inspecting a PCB has several disadvantages. First, when a component has the same color as the background, the component is effectively camouflaged relative to the PCB, making it difficult for the computer to determine where the edges of a component are placed and thereby provide the location of the component. This reduces the accuracy of the optical inspection since a misplaced component may not be reported as a defect. Alternatively, since the component may not be accurately traced, it may be reported as a defect even though it is correctly placed. Furthermore, since existing optical inspection devices view the components from directly above, it is difficult to determine whether components are properly seated (vertically) even when they are in a proper location. Finally, since each manufacturer may manufacture electrically identical devices or boards in different colors, when components or boards of different colors are loaded into a PCB system, prior art defect detection methods may erroneously "detect" defects when no defects are present.
Some of the aforementioned deficiencies can be overcome by using laser or X-ray imaging. However, because laser and X-ray imaging techniques illuminate and reconstruct only one point at a time, they are slow and very expensive. Alternatively, some manufactures of optical PCB inspection equipment use multiple cameras to create a "stereo" image of the PCB. Stereo imaging can detect a vertically misplaced component, but requires an additional camera and still has trouble detecting component edges when the components are the same color as the PCB. Stereo imaging also has trouble detecting small components.
In order to overcome the disadvantages of the existing methods of PCB inspection, it would be advantageous to have an apparatus, system and method of optically inspecting PCBs for manufacturing defects, such as alignment errors, missing components, embedded components, skewed components, and foreign objects. Ideally, the defect detection system should be able to reliably detect and assess the location of components having the same color as the PCB or other background, and detect when components are not properly vertically seated on the PCB. In addition, it would also be advantageous for such a system to utilize only a single camera and not require extensive lighting, thereby reducing system costs. The present invention provides such a system and method.